The referenced U.S. Pat. No. 4,940,476, Buck, the inventor hereof, describes a filter especially adapted to clean exhaust gases from Diesel engines. A housing is provided in which a support element is located on which the filter material is secured. The support element is a perforated tube, closed at one end. The exhaust gases are guided radially through the tube and hence through the filter material. The filter material is a textile structure formed of knitted mineral fibers. Use of mineral fibers has the advantage that they can readily accept the high temperature in the exhaust gas from internal combustion engines. Using a knitted fabric, rather than a woven fabric, has the advantage that the filter material is self-supporting and is not shifted in position due to variations in the flow of exhaust gases, and particularly pulses and intermittent rapid oscillations in the flow, endemic in the exhaust gases from internal combustion engines.
Filters of this type eventually become clogged by carbon particles which deposit on the filter material.
It is known to clean soot from various surfaces, and also from a filter of this type, by burning off the accumulated carbon particles. Excess oxygen, usually present in the exhaust gases, is used for the combustion. The temperature within the filter must be increased substantially in order to obtain such combustion. It has been proposed to heat the exhaust gases by passing them past an electrical heating element, through which a current in the order of about 120 A is passed, at voltages between 12 to 24 V. Merely heating the exhaust gases by a constant heat supplying element involves dangers, however, since the hot gases may then reach a temperature which is so high that the resistance heating element itself is endangered. The quantity of exhaust gases varies over time, and particularly when coming from Diesel engines, is subject to peaks and pulses. If the gas stream is not sufficiently strong to remove the heat from the resistance heating element, hot spots may occur which cause destruction of the heating element itself.